Create PULL_REQUEST_TEMPLATE

docs/PULL_REQUEST_TEMPLATE

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+<!--
+# Adding new features
+
+If you want to add a new feature, consider asking about it before implementing it to
+make sure it is within the scope of the project. Of course you are free to create the
+pull request directly but it is not guaranteed it is going to be accepted.
+
+# Style
+
+- Only use tabs for indentation, not spaces.
+- Do not remove or add optional characters from other lines (such as colons or new line
+  characters) as it can make the commit needlessly big, and create conflicts with other
+  changes.
+-->